tztab — time zone adjustment table for date and ctime()
file describes the differences between Coordinated Universal Time
(UTC) and local time.
Several local areas can be represented simultaneously
with historical detail.
consists of one or more time zone adjustment entries.
The first line of the entry contains a unique string
that may match the value of the
string in the user's environment.
The format is
is the time zone name or abbreviation,
is the difference in hours from UTC,
is the name or abbreviation of the "Daylight Savings" time zone.
Fractional values of
are expressed in minutes preceded by a colon.
Each such string will start with an alphabetic character.
The second and subsequent lines of each entry
details the time zone adjustments for that time zone.
The lines contain seven fields each.
The first six fields specify the first minute in which the time zone
adjustment, specified in the seventh field, applies.
The fields are separated by spaces or tabs.
The first six are integer patterns that specify the
day of the month (1-31),
month of the year (1-12),
and day of the week (0-6, with 0=Sunday).
The minute, hour, and month of the year must
contain a number in the (respective) range indicated above.
The day of the month, year, and day of the week
can contain a number as above or two numbers
separated by a minus (indicating an inclusive range).
Either the day of the month or the day of the week field
must be a range, the other must be simple number.
The seventh field is a string
that describes the time zone adjustment in its simplest form:
is an alphabetic string giving the time zone name or abbreviation, and
is the difference in hours from UTC.
must match either the
field or the
field in the first line of the time zone adjustment entry.
is shown in minutes.
Comments begin with a
column, and include all characters up to a newline.
Comments are ignored.
International Code Set Support
Single-byte character code sets are supported.
The time zone adjustment table
for the Eastern Time Zone in the United States is:
0 3 6 1 1974 0-6 EDT4
0 3 22-28 2 1975 0 EDT4
0 3 24-30 4 1976-1986 0 EDT4
0 3 1-7 4 1987-2038 0 EDT4
0 1 24-30 11 1974 0 EST5
0 1 25-31 10 1975-2038 0 EST5
Normally (as indicated in the first line)
Eastern Standard Time is five hours earlier than UTC.
During Daylight Savings time, it changes to a 4 hour difference.
The first time Daylight Savings Time took effect (second line)
was on January 6, 1974 at 3:00 a.m., EDT.
Note that the minute before was 1:59 a.m., EST.
The change back to standard time took effect (sixth line)
on the last Sunday in November of the same year.
At that point, the time went from 1:59 a.m., EDT
to 1:00 a.m., EST.
The transition to Daylight Savings Time since then
has gone from the last Sunday in February (third line)
to the last Sunday in April (fourth line)
to the first Sunday in April (fifth line).
The return to standard time for the same period
has remained at the last Sunday in October (seventh line).
was developed by HP.